analytics: Add backend for messages read over time graph.

This commit includes changes in the api /json/analytics/chart_data to
send data for the newly added graph, as well as tests.

analytics/views.py

@@ -236,6 +236,12 @@ def get_chart_data(request: HttpRequest, user_profile: UserProfile, chart_name:
                              {str(id): name for id, name in Client.objects.values_list('id', 'name')}}
         labels_sort_function = sort_client_labels
         include_empty_subgroups = False
+    elif chart_name == 'messages_read_over_time':
+        stats = [COUNT_STATS['messages_read::hour']]
+        tables = [aggregate_table, UserCount]
+        subgroup_to_label = {stats[0]: {None: 'read'}}
+        labels_sort_function = None
+        include_empty_subgroups = True
     else:
         raise JsonableError(_("Unknown chart name: %s") % (chart_name,))